[Bug c++/80488] New: Erroneous error "lambda-expression in template argument"

["eric.niebler at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80488

            Bug ID: 80488
           Summary: Erroneous error "lambda-expression in template
                    argument"
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

Compile with -std=gnu++1z:

  template <auto> using x = void;
  using Y = x<+[]{}>;

The above code creates a lambda and then applies unary operator +, forcing it
to decay to a function pointer. Since that's constexpr, it should be a valid
template argument.

clang-trunk accepts this code.